^^ Several ecological attributes were selected from the diatom reports in the appendix and 



modal categories of these attributes were extracted to characterize water quality tendencies at the 

 three sites on Little Dry Creek (Table 5). 



Modal categories were very similar for the two upstream sites. Most diatoms at these 

 sites were highly motile, alkaliphilous, eutraphentic species that indicate waters with 25-70% 

 saturation of dissolved oxygen and 4-13 mg/L BOD5 (Van Dam et al. 1994). BOD is a measure 

 of organic loading in a body of water. Additional organic loading may result in more growth of 

 aquatic bacteria, fungi, and heterotrophic algae and in lower concentrations of dissolved oxygen. 

 The intermediate level of BOD loading (saprobity) at the two upstream sites is less than what 

 would be discharged by a sewage treatment plant, but more than what would be present in a 

 stream where biodegradable organic matter has been mostly decomposed. 



Most diatoms at Cohagen were facultative nitrogen heterotrophs that require only 

 moderate (above 50% saturation) dissolved oxygen. The modal category for nitrogen uptake and 

 oxygen demand at the Uall Creek site was "not classified". 



Modal categories at Van Norman were significantly different from those at the two 

 upstream sites (Table 5). Most diatoms at Van Norman were only moderately (not highly) 

 motile an indicated circumneutral (not alkaline) waters. Most diatoms here were nitrogen 

 autotrophs that require inorganic nitrogen (nitrates and ammonia) rather than preformed organic 

 matter as a source of nitrogen. The majority of diatoms at Van Norman requires continuously 

 high concentrations of dissolved oxygen and indicates smaller amounts of BOD5 (2-4 mg/L). 

 Most diatoms at Van Norman tolerate a wide range of trophic conditions ranging from 

 oligotrophic to eutrophic (Table 5). 



References 



APHA. 1998. Standard Methods for the Examination of Water and Wastewater. 20"^ Edition. American Public 

 Health Association, Washington, D.C. 



Bahls, L.L. 1979. Benthic diatom diversity as a measure of water quality. Proceedings of the Montana 

 Academy of Sciences 38:1-6. 



